We propose to investigate the mechanisms by which mutations in the ras oncogene result in neoplastic transformation, using Drosophila as a model system. We have established Drosophila strains that contain an activated ras gene in their germline; the expression of this gene results in altered patterns of growth and differentiation of various tissues of the developing fly, depending on the time of induction of the transformed gene. In this application we propose a dual approach to further our understanding of the role of the ras-encoded protein in the physiology of the cell. First we will examine in detail at the cellular level the aberrant phenotypes caused by ras overexpression using immunofluorescent and electron microscopical techniques. In particular, we will study the structural alterations that occur in the developing eye imaginal discs of activated ras-transformed flies using specific antibodies that stain photoreceptor cells; similar alterations will be analyzed during late pupal stages by electron microscopy of serial sections through the developing eye. This information will allow us to identify the exact nature of the biological processes affected by activated ras expression and will make possible to establish hypotheses on the role of ras protein in growth and differentiation. The second approach will be to isolate second- site mutations that reverse or enhance the phenotypic effect of activated ras protein expression. These loci will be cloned using hybrid dysgenesis and/or chromosomal walking. The characterization of the protein products encoded by these modifier genes will afford the identification of factors that interact with ras protein at the metabolic level and will therefore offer important clues to understand the physiological function of ras.